The water-stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient. mainly polysaccharides, (b), temporary, roots and fungal hyphae, and (c) persistent. resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers. The effectiveness of various binding agents at different stages in the structural organization of aggregates is described and forms the basis of a model which illustrates the architecture of an aggregate. Roots and hyphae stabilize macroaggregates, defined as > 250 pm diameter; consequently, macroaggregation is controlled by soil management (i.e. crop rotations), as management influences the growth of plant roots, and the oxidation of organic carbon. The water-stability of microaggregates depends on the persistent organic binding agents and appears to be a characteristic of the soil, independent of management.

The root system of ryegrass was more efficient than that of white clover in stabilizing aggregates of Lemnos loam because ryegrass supported a larger population of vesicular-arbuscular mycorrhizal hyphae in the soil. Electron micrographs show that the hyphae were covered with a layer of amorphous material, probably polysaccharide, to which clay particles appear firmly attached.

No abstract

In many soils, roots and fungal hyphae, especially those of vesicular arbuscular mycorrhizal (VAM) fungi, stabilize macroaggregates (> 250 #m diameter); organic residues, bacteria, polysaccharides and inorganic materials stabilize microaggregates (< 250 #m). This review discusses the factors (including other organisms) which affect VAM hyphae and their extracellular polysaccharides in soil, and the subsequent effect on stability of aggregates. The review also discusses the possible role of other organisms, including ectomycorrhizal fungi, in the stability of soil, and suggests future research.